There are a couple of ways to overwinter geraniums. One is to simply take them indoors. Pruning them back won't hurt them. In fact, it might be easier to find room for them indoors if you cut them back. Find a warm sunny spot indoors and they should grow well. Another way is to cut them back and set them in a garage or basement where the temperatures remain above freezing and just leave them alone. Don't water them. They will go into a dormant state. In early spring begin watering them again and they should sprout new growth. If you have more than one geranium, you might try both methods to see which works best for you. Enjoy your geraniums!